Adiabatic Salt Electric Energy Storage

ABSTRACT

Efficient energy storage is provided by using a working fluid flowing in a closed cycle including a ganged compressor and turbine, and capable of efficient heat exchange with heat storage fluids on a hot side of the system and on a cold side of the system. This system can operate as a heat engine by transferring heat from the hot side to the cold side to mechanically drive the turbine. The system can also operate as a refrigerator by mechanically driving the compressor to transfer heat from the cold side to the hot side. Heat exchange between the working fluid of the system and the heat storage fluids occurs in counter-flow heat exchangers. In a preferred approach, molten salt is the hot side heat storage fluid and water is the cold side heat storage fluid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application 61/339,577, filed on Mar. 4, 2010, entitled “Adiabatic salt energy storage”, and hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to energy storage.

BACKGROUND

Large scale energy storage is of considerable interest for power generation and distribution systems, to assist with exploitation of capricious energy sources such as wind and solar. At the moment, the main technology in wide use for reversibly storing electric power is hydropumping—drawing electricity off the grid to pump water uphill and then letting the water back down through power turbines later on. Hydropumping is highly efficient (about 80%) but suffers from (1) the need to allocate land to build dams and lakes, (2) the need for mountains, which aren't always available nearby, and (3) the need for water.

Recent developments in solar energy have revealed the substantial cost effectiveness of storing heat in tanks of molten salt for later use in generating electricity, by means of steam turbines, when the sun isn't shining. However, these storage facilities are adapted to store solar thermal energy, and are therefore not directly applicable to the storage of wind energy, which is mechanical energy as opposed to thermal energy. Molten salt has also been used as a primary coolant in nuclear reactors. Another approach for energy storage is considered in US 2010/0251711, where hot and cold storage tanks are employed in connection with heat storage.

However, efficiency is critical for energy storage, and it is especially critical for large scale energy storage. Therefore, it would be an advance in the art to provide energy storage having improved efficiency, especially for capricious sources of mechanical energy (e.g. wind energy).

SUMMARY

Improved energy storage is provided by using a working fluid flowing in a closed cycle including a ganged compressor and turbine, and capable of efficient heat exchange with heat storage fluids on a hot side of the system and on a cold side of the system. This system can operate as a heat engine by transferring heat from the hot side to the cold side to mechanically drive the turbine. The system can also operate as a refrigerator by mechanically driving the compressor to transfer heat from the cold side to the hot side. Heat exchange between the working fluid of the system and the heat storage fluids occurs in counter-flow heat exchangers.

Preferably, the hot side and cold side heat storage fluids each have a corresponding pair of storage tanks, where heat transfer to/from a heat storage fluid entails flow of the heat storage liquid between its two corresponding storage tanks. In a preferred approach, molten salt is the hot-side heat storage fluid and water is the cold-side heat storage fluid.

This approach provides numerous significant advantages. The use of the same compressor and turbine for both storage and retrieval provides substantial cost savings relative to approaches where storage and retrieval are performed in separate machinery. This cost savings is expected to be extremely significant, because the cost of the compressor and turbine (or equivalent machinery) is expected to be the most significant capital expense for a large scale energy storage plant. Molten salt provides numerous advantages as a thermal energy storage medium, such as low vapor pressure, lack of toxicity, low chemical reactivity with typical steels, and low cost. The low vapor pressure of molten salt is a highly significant safety advantage, as can be appreciated by considering hypothetically the use of steam as an energy storage medium in a large scale (e.g., 1 GW) thermal energy storage facility. A steam explosion from such a facility could have an explosive force on the order of thousands of tons of TNT. Using a closed loop for the working fluid advantageously increases cold-side heat transfer rates, allows a broader selection of working fluids, allows for operation at elevated cold-side pressure, improves efficiency, and reduces the risk of turbine damage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment of the invention.

FIG. 2 shows operation of the example of FIG. 1 in a heat engine mode that uses heat energy to provide mechanical work.

FIG. 3 shows operation of the example of FIG. 1 in a refrigerator mode that uses mechanical work to store heat energy.

FIG. 4 shows an idealized thermodynamic Brayton cycle that relates to operation of embodiments of the invention.

FIG. 5 show plots of compressor efficiency vs. number of compressor stages.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary embodiment of the invention. In this example, a working fluid (schematically referenced as 108) flows in a closed cycle that includes, in sequence, a compressor 102, a first heat storage unit 110, a turbine 104, and a second heat storage unit 120. Compressor 102 and turbine 104 are ganged on a common mechanical shaft 106 such that they rotate together. Heat storage units 110 and 120 are both capable of exchanging heat with working fluid 108. For ease of illustration, pipes for defining the flow paths of fluids (e.g., working fluid 108) are not shown on FIGS. 1-3. Suitable pipes for the working fluid and heat storage fluids described herein are known in the art. As described in greater detail below, this apparatus is capable of operating as a heat engine (to provide mechanical work from heat) or as a refrigerator (to use mechanical work to store heat).

The purpose of heat storage units 110 and 120 is to provide stored heat to working fluid 108 and to remove heat from working fluid 108 for storage. It can be helpful to refer to first heat storage unit 110 as the hot-side heat storage unit, and to refer to second heat storage unit 120 as the cold-side heat storage unit. This terminology can be understood by noting that hot-side heat storage unit 110 adds heat to working fluid 108 at the same point in the cycle that combustion of fuel adds heat to air in a conventional jet engine. Thus, it can be helpful to regard hot-side heat storage unit 110 as being analogous to the fuel in a jet engine, when the apparatus is operating as a heat engine.

Heat storage units 110 and 120 preferably have several features to improve efficiency, as shown on FIGS. 1-3. First heat storage unit 110 preferably includes a first hot heat storage tank 112H, a first cold heat storage tank 112C, a first heat storage fluid 118 capable of flowing between tanks 112H and 112C to store or release heat, and a first counter-flow heat exchanger 116. In counter-flow heat exchanger 116, it is important that working fluid 108 and first heat storage fluid 118 flow in opposite directions, as shown. First heat storage unit 110 also includes a valve 114 that can switch connections between heat exchanger 116 and tanks 112H, 112C as needed for the heat engine and refrigerator modes.

Second heat storage unit 120 preferably includes a second hot heat storage tank 122H, a second cold heat storage tank 122C, a second heat storage fluid 128 capable of flowing between tanks 122H and 122C to store or release heat, and a second counter-flow heat exchanger 126. In counter-flow heat exchanger 126, it is important that working fluid 108 and second heat storage fluid 128 flow in opposite directions, as shown. Second heat storage unit 120 also includes a valve 124 that can switch connections between heat exchanger 126 and tanks 122H, 122C as needed for the heat engine and refrigerator modes.

Counter-flow heat exchangers 116 and 126 can be designed according to known principles to reduce entropy generation in the heat exchangers to negligible levels compared to the compressor entropy generation. The basic idea is to have very small temperature differences between any two fluid elements that are exchanging heat, thereby reducing entropy production (and eliminating it entirely in the idealized case).

The heat storage tanks are thermally insulated tanks that can hold a suitable quantity of the relevant heat storage fluid. In other words, the heat storage fluids are the medium of heat storage. Liquids are preferred over solids or gases because of the need for extremely rapid exchange of large amounts of heat by convective counterflow. They also allow for relatively compact storage of large amounts of energy. For example, the size of each storage unit (i.e. 110 and 120 on FIG. 1) for a 1 GW plant operating for 12 hours should be roughly 20 medium-size oil refinery tanks. Each heat exchanger (i.e. 116 and 126 on FIG. 1) should be roughly the size of a large steam locomotive boiler.

On the hot side, it is preferred that the heat storage fluid (i.e., fluid 118) be a molten salt or mixture of molten salts. A preferred molten salt is a eutectic (i.e. lowest melting point) mixture of sodium nitrate and potassium nitrate. However, any salt or salt mixture that is liquid over the operating temperature range can be employed. Such molten salts can provide numerous advantages, including low vapor pressure (which is important for safety), melting point below the creep temperature of steels, low corrosiveness, low capacity to dissolve iron and nickel, chemical stability, lack of toxicity, and low cost.

On the cold side, it is preferred that the heat storage fluid (i.e., fluid 128) be liquid water. It is important to ensure that no steam is present on the cold side, because the presence of steam creates a significant explosion hazard. Thus, 100° C. is an upper limit for the temperature of heat storage fluid 128 if water is used. As will be seen below, efficiency is improved by increasing the temperature difference at which the system operates. Accordingly, in some preferred embodiments, a mixture of water and one or more antifreeze compounds (e.g., ethylene glycol, propylene glycol and glycerol) is employed to increase the cold side temperature range to greater than 100° C. (e.g., −30° C. to 100° C.).

The example of FIG. 1 also preferably includes a radiator 130 for dissipating waste heat generated by operation of the apparatus. Preferably, the radiator is coupled to the second hot heat storage tank 122H, as shown. However, practice of the invention does not depend critically on the location of the radiator, because waste heat can also be removed at other points in the cycle.

Before describing further preferred features of some embodiments of the invention, it will be helpful to consider the heat engine and refrigerator modes of this apparatus, in connection with FIGS. 2-4. A idealized thermodynamic Brayton cycle is shown on FIG. 4 as a pressure-volume diagram.

FIG. 2 shows operation of the example of FIG. 1 in a heat engine mode that uses heat energy to provide mechanical work. Here it is assumed that the hot-side storage tanks 112H and 112C have substantially different fluid temperatures (e.g., as would result from prior operation of the apparatus to store energy). Working fluid 108 at the input of compressor 102 is represented by point 408 on FIG. 4. Compression of working fluid 108 moves the system to point 402 on FIG. 4. Heat is added by heat storage unit 110 to move the system from 402 to 404 on FIG. 4. More specifically, valve 114 provides connections as shown such that heat storage fluid flows from tank 112H to tank 112C through heat exchanger 116, thereby providing heat to working fluid 108. Working fluid 108 expands in turbine 104 to move the system from 404 to 406 on FIG. 4. Mechanical energy is provided by the apparatus in this mode, because the work released by expanding from 404 to 406 on FIG. 4 is greater than the work required to compress from 408 to 402 on FIG. 4.

Importantly, the thermodynamic cycle of FIG. 4 is closed by connecting the exhaust of turbine 104 to the input of compressor 102 through the cold-side heat storage unit 120. Heat is removed from working fluid 108 by heat storage unit 120 to move the system from 406 to 408 on FIG. 4. More specifically, valve 124 provides connections as shown such that heat storage fluid flows from tank 122C to tank 122H through heat exchanger 126, thereby storing heat provided by working fluid 108. This step can be understood as storing the heat energy present in the (hot) exhaust from turbine 104. Hot-side heat storage unit 110 and cold-side heat storage unit 120 have comparable total heat capacity. The need for this can be appreciated in connection with the generation mode of FIG. 2, where it is apparent that cold-side heat storage unit 120 stores a fraction of the heat stored in hot-side heat storage unit 110 (i.e., the fraction of the stored hot-side heat that ends up in the exhaust from turbine 104).

FIG. 3 shows operation of the example of FIG. 1 in a refrigerator mode that uses mechanical work to store heat energy. Working fluid 108 at the input of compressor 102 is represented by point 406 on FIG. 4. Compression of working fluid 108 moves the system to point 404 on FIG. 4. Heat is removed by heat storage unit 110 to move the system from 404 to 402 on FIG. 4. More specifically, valve 114 provides connections as shown such that heat storage fluid flows from tank 112C to tank 112H through heat exchanger 116, thereby removing heat from working fluid 108. Working fluid 108 expands in turbine 104 to move the system from 402 to 404 on FIG. 4. Mechanical energy must be provided to the apparatus in this mode, because the work released by expanding from 402 to 408 on FIG. 4 is less than the work required to compress from 406 to 404 on FIG. 4.

Importantly, the thermodynamic cycle of FIG. 4 is closed by connecting the exhaust of turbine 104 to the input of compressor 102 through the cold-side heat storage unit 120. Heat is added to working fluid 108 by heat storage unit 120 to move the system from 408 to 406 on FIG. 4. More specifically, valve 124 provides connections as shown such that heat storage fluid flows from tank 122H to tank 122C through heat exchanger 126, thereby providing heat to working fluid 108. This step can be understood as warming up the (cold) exhaust from turbine 104.

From the preceding description, it is apparent that in either mode of operation, two of the storage tanks 112H, 112C, 122H, and 112C will be feeding heat storage fluid to the system, and the other two tanks will be receiving heat storage fluid. The feed tanks set their own temperatures. The receiving tanks see fluid temperatures that depend on how the system is operating—i.e., its loads and/or power input. Ideally, the receiving tank fluid temperatures are set by the Brayton cycle conditions, but in practice there will be deviations from these conditions, and the pressure ratio varies in response to system demand.

A system controller (not shown) controls system parameters in order to approximately match the ideal temperature conditions. Suitable system parameters include but are not limited to: the flow rate of first heat storage fluid 118, the flow rate of second heat storage fluid 128, and operating parameters of compressor 102 and turbine 104 such as turbine stator blade positions. Because of entropy creation within the system, it will not be possible to match the ideal temperature conditions exactly, and at least one of the four tank temperatures will be too high. The purpose of radiator 130 is to reject this waste heat to the environment as part of system control. Suitable techniques for controlling systems as described herein are known in the art.

Some principles of the present invention can be better appreciated in connection with a specific example where hot-side heat storage fluid 118 is a molten salt and cold-side heat storage fluid 128 is water. In this example, there is a water side and a salt side, each characterized by two temperatures. However, these 4 temperatures are not independent of each other. Each salt temperature is the product of the corresponding water temperature and a factor that depends on the compressor pressure ratio (numerically, this factor is typically about 2). Thus, in steady state operation, there are only two independent temperatures. The water temperatures need to be in the liquid range for water (at 1 atmosphere) for safety, and the salt temperatures need to be in the liquid range for the relevant salt, and be at a temperature range that structural steels can handle. Fortunately, salts that are molten at temperatures on the order of 450-700 K are known, and such temperatures are well below typical steel melting or creep temperatures.

To better appreciate the present approach, it is helpful to note that it is possible to perform energy storage and retrieval without using a closed cycle for working fluid 108. More specifically, the cold-side heat storage unit 120 could be removed from FIG. 1, thereby opening the cycle such that the compressor input is provided by the environment, and the turbine exhausts to the environment.

However, this open-cycle approach has numerous and severe disadvantages. The open-cycle approach entails employing atmospheric air as the cold-side heat reservoir. This automatically precludes the use of any working fluid other than air. It also precludes the use of counterflow heat exchange to minimize entropy production. It also exposes the system to environmental dangers, for example humidity fluctuations that could cause condensation or even freezing of water in the turbine operating in refrigerator mode, with total destruction of the turbine as the likely result.

A particularly important modification of the working fluid allowed by a closed cycle is pressurization. This enables the input pressure to compressor 102 to be higher than atmospheric pressure. It is helpful to consider the minimum pressure (P_(min)) of working fluid 108 in its closed cycle. The minimum pressure is typically found on the cold side of the apparatus (e.g., at the input to compressor 102). Although P_(min) can be as low as 1 atmosphere (atm), it is preferred for P_(min) to be about 10 atmospheres or greater.

This increase in power density provided by a high-pressure working fluid can be extremely significant. A storage turbine at 1 atm pressure generates about 1/10 the power of a combustion turbine of the same size. This can be seen by comparing the exhaust temperatures. For example, a large commercial power gas turbine has an output of 256 megawatts, a compression ratio of 15.3 and an exhaust temperature of 608° C. (i.e. 578° C. greater than the intake temperature). For a storage turbine based on air, which might have a compression ratio of 14 and an exhaust temperature rise (retrieval step) of 75° C., the same size as the above commercial power turbine and flowing the same amount of working fluid (643 kg/sec), the resulting power is (256 MW) (75° C.)/(578° C.)=33.2 MW. This is absurdly low for such a large machine.

To put this problem in perspective, the throat intake speed of industrial gas turbines is typically a significant fraction of the sound speed in air (e.g., Mach 0.5). Since sea level air has a mass density of 1.22 kg/m³ and a sound speed of 343 m/s, the throat area required to accommodate the mass flow is about 3 m². The power required merely to accelerate the air mass in question to Mach 0.5 is about 9.5 MW. Some of this power can be recovered from exhaust hydrodynamics, but not all, and the lost part is comparable to the energy one is trying to extract.

Thus it is important that the power output of the turbine of a given size be substantially raised. This can be done by raising the ambient pressure of the working fluid. If, for example, the pressure is raised to 10 atmospheres, something that steel can accommodate easily, the power output becomes 10 times what it was before, which is an amount comparable to that generated by a combustion gas turbine of the same size. The pressures and temperatures in question also feature in modern supercritical steam plants, so the steel is expected to be able to take the stress. The elevated working fluid density should also help raise the compressor efficiency, although the exact amount is difficult to estimate accurately. Water (i.e., a dense fluid) can be pumped uphill with 90% efficiency using Francis turbines. This high efficiency is what makes hydropumping the leading energy storage technology at the moment.

The closed loop also enables one to conserve momentum, as in a wind tunnel. This becomes increasingly important as the mass of the fluid rises, for then the total fluid kinetic energy passing by a point per second can become comparable to the power one is trying to store or extract. In a closed circuit this energy is automatically conserved (except for friction losses at the walls) so it doesn't reduce efficiency, but in an open circuit, where kinetic energy gets lost to the environment, it does reduce efficiency.

To better appreciate some further preferred embodiments, it is helpful to provide some results from an analysis of the Brayton cycle of FIG. 4. For adiabatic compression of a gas having temperature T₀ and pressure P₀ to a pressure P₁, the resulting temperature T₁ is given by

$\begin{matrix} {{T_{1} = {T_{0}\left( {P_{1}/P_{0}} \right)}^{\frac{\gamma - 1}{\gamma}}},} & (1) \end{matrix}$

where γ is the heat capacity ratio (i.e., C_(p)/C_(v)) of the gas. The heat dumped to the environment per mole of working fluid compressed (Q_(dump)) is given by

$\begin{matrix} {{Q_{dump} = {\left( {1 - \eta_{c}} \right){\frac{{RT}_{e}}{\gamma - 1}\left\lbrack {1 - \left( {P_{0}/P_{1}} \right)^{\frac{\gamma - 1}{\gamma}}} \right\rbrack}}},} & (2) \end{matrix}$

where R is the ideal gas constant, η_(c) is the compressor efficiency, and T_(e) is the environment temperature. It is assumed that the compressor is the dominant source of entropy production in the cycle. This assumption is reasonable in view of the use of counter-flow heat exchangers and the high efficiencies provided by turbines in practice. The energy stored per mole of working fluid compressed (E_(store)) is given by

$\begin{matrix} {{E_{store} = {\frac{\gamma}{\gamma - 1}R\; \Delta \; {T\left\lbrack {1 - \left( {P_{0}/P_{1}} \right)^{\frac{\gamma - 1}{\gamma}}} \right\rbrack}}},} & (3) \end{matrix}$

where ΔT is the temperature difference between the hot and cold storage tanks (e.g., 112H and 112C). The thermodynamic efficiency of energy storage (η_(store)) is given by

$\begin{matrix} {\eta_{store} = {{1 - \frac{Q_{dump}}{E_{store}}} = {1 - {\frac{\left( {1 - \eta_{c}} \right)}{\gamma}{\frac{T_{e}}{\Delta \; T}.}}}}} & (4) \end{matrix}$

For a numerical example, let P₁/P₀=14, η_(c)=0.9, T_(e)=300 K, ΔT=150 K and γ=1.4. The resulting storage efficiency is η_(store)=0.857. The efficiency of retrieval is the same as for storage, so the total efficiency for storage+retrieval is η_(store) ².

From these results, several further preferred features may be understood. Although air can be employed as a working fluid, a preferred working fluid is Argon. Argon is inexpensive, and has better properties than air. More specifically, γ for Argon is 1.66 and γ for air is 1.4, so Argon is seen to improve the efficiency given by Eqn. 4. Commonly employed working fluids in conventional refrigerators, such as ammonia and freon, are not preferred working fluids in this context, because drops of their liquid phase may form in operation and damage the turbine blades.

The effect of the use of Argon instead of air as the working fluid can be better appreciated in view of some compressor design considerations. The compressor is the dominant source of inefficiency in the present apparatus. Axial compressors, (e.g. those in jets and as shown on FIGS. 1-3) tend to be the most efficient kind of compressor, particularly in applications requiring large volume flows. The fundamental limit of efficiency per stage in an axial compressor is about η_(s)=0.9. The overall compressor efficiency degrades with stage number n according to:

$\begin{matrix} {{\eta_{c} = \frac{r^{{n{({\gamma - 1})}}/\gamma} - 1}{r^{{{n{({\gamma - 1})}}/\eta_{s}}\gamma} - 1}}{where}{r = {\left\lbrack \frac{P_{1}}{P_{0}} \right\rbrack^{1/n}.}}} & (5) \end{matrix}$

FIG. 5 shows plots of η_(c)(n) for η_(s)=0.9 (top curve) and η_(s)=0.8 (bottom curve). The compression ratio per stage is taken to be r=1.4, and the specific heat ratio is taken to be γ=1.4. For P₁/P₀=14 as in the preceding example, the number of stages required is 7.8 (which rounds to 8). This number of stages degrades the overall compressor efficiency to 0.86 and reduces the storage-step efficiency to η_(store)=0.80. The retrieval-step efficiency is the same, so the round-trip storage efficiency is the square of this number, or 0.64.

Thus there is a significant premium in increasing the stage efficiency even by a tiny amount and thereby reducing the number of stages. Substituting Ar for air as the working fluid, for example, increases the specific heat ratio to γ=1.66, reduces the overall compression ratio required from 14 to 6.7, and thus reduces the number of stages to 5.6 (which rounds to 6). The storage-step efficiency then rises to 0.84, which gives 0.71 when squared.

It is also clear from Eqn. 4 that there is a significant efficiency advantage in maximizing the temperature difference ΔT_(H) between the tanks on the hot side of the circuit. This is related by the Brayton cycle condition to the temperature difference ΔT_(C) between the tanks on the cold side of the circuit by

ΔT _(H)=(P ₁ /P ₀)^((γ−1)/γ) ΔT _(C).   (6)

For P₁/P₀=14, γ=1.4 (i.e., air), and ΔT_(C)=75 K, the resulting ΔT_(H) is about 150K (more specifically, it is 159 K). The value for ΔT_(C) in this example is a conservative liquid range for water. It is highly undesirable to pressurize the water to allow temperatures greater than 100° C., on account of the extreme explosion danger thereby created. Thus the only practical way to increase this range is extend the cold side to below room temperature. One can obtain a further 25 K by going down to the freezing point of water and a further 30 K if antifreeze is added as described above. Assuming ΔT_(C)=130 K, the hot-side temperature difference then becomes ΔT_(H)=276 K which gives a corresponding storage efficiency of η_(store)=0.91.

In the preceding two examples, the effects of using Argon as the working fluid and of increasing the cold-side temperature difference were considered separately for ease of explanation. These approaches for improving efficiency can be practiced simultaneously, and can also be practiced in combination with any other ways of improving efficiency (e.g., operating at higher pressures). Preferably, efficiency is maximized by making use of all available methods of increasing efficiency. For example, the choice of working fluid can be considered and optimized in combination with compressor/turbine optimization. 

1.-14. (canceled)
 15. A method for storing and releasing energy, the method comprising: (a) increasing the pressure of a working fluid operating in a closed cycle from a first pressure to a second pressure with the aid of a compressor, thereby increasing the temperature of the working fluid; (b) using a first heat storage unit downstream of the compressor and in thermal communication with the working fluid for (i) in a storing mode, removing heat from the working fluid and decreasing the temperature of the working fluid, which decrease in temperature is at substantially the second pressure, or (ii) in a releasing mode, supplying heat to the working fluid and increasing the temperature of the working fluid, which increase in temperature is at substantially the second pressure; (c) decreasing the pressure of the working fluid from the second pressure to the first pressure with the aid of a turbine, thereby decreasing the temperature of the working fluid; (d) using a second heat storage unit downstream of the turbine and in thermal communication with the working fluid for (i) in a storing mode, supplying heat to the working fluid and increasing the temperature of the working fluid, which decrease in temperature is at substantially the first pressure, or (ii) in a releasing mode, removing heat from the working fluid and decreasing the temperature of the working fluid, which decrease in temperature is at substantially the first pressure, wherein, during use, entropy generation in said first and second heat storage units is less than the entropy generated by said compressor, and wherein the working fluid flows in the same general direction in the closed cycle when in the storing mode as when in the releasing mode.
 16. The method of claim 15, wherein said second heat storage unit has a range of operating temperatures that is lower than the range of operating temperatures of the first heat storage unit.
 17. The method of claim 15, wherein said first and second heat storage units have comparable total heat capacities.
 18. The method of claim 15, wherein the first heat storage unit comprises a heat storage fluid that includes molten salt.
 19. The method of claim 18, wherein the molten salt comprises sodium nitrite and/or potassium nitrate.
 20. The method of claim 15, wherein the first and/or second heat storage unit comprises a heat storage fluid that is liquid at a range of operating temperatures of said heat storage fluid.
 21. The method of claim 15, wherein the second heat storage unit comprises a heat storage fluid that is at ambient pressure.
 22. The method of claim 15, wherein a radiator is operationally coupled to the first heat storage unit.
 23. A system for storing and releasing electrical energy, comprising an electrical energy input and an electrical energy output, wherein the system is capable of storing and releasing electrical energy using two heat exchangers, each heat exchanger comprising a thermal storage fluid that directs thermal energy into or extracts thermal energy from a circulatory fluid flow path, wherein the circulatory fluid flow path comprises a working fluid that that flows in the same general direction in the circulatory fluid flow path when storing electrical energy as when releasing electrical energy, and wherein the system comprises a controller that is programmed to regulate (i) a temperature difference between any two thermally coupled fluid elements of the heat exchangers, and/or (ii) one or more fluid properties of the fluid elements of the heat exchangers such that entropy generation in the heat exchangers is minimized during operation of the system.
 24. The system of claim 23, wherein the electrical energy input is stored as thermal energy.
 25. The system of claim 23, wherein the electrical energy output is extracted from thermal energy.
 26. The system of claim 23, wherein the electrical energy use comprises a transformation to thermal energy.
 27. A method for storing and releasing energy, the method comprising: (a) providing a system comprising a closed cycle comprising, in sequence, a compressor, a first heat storage unit, a turbine, and a second heat storage unit, wherein the first and second heat storage units comprise fluid elements that exchange heat with a working fluid flowing through the closed cycle; and (b) alternately and sequentially operating the system in a refrigerator mode and a heat engine mode, wherein in the refrigerator mode, mechanical work is used to transfer thermal energy from the second heat storage unit to the first heat storage unit, wherein, in the heat engine mode, thermal energy transferred from the first heat storage unit to the second heat storage unit is used to provide mechanical work, and wherein the working fluid flows in the same general direction in the closed cycle when operating in the refrigerator mode as when operating in the heat engine mode.
 28. The method of claim 27, further comprising controlling a temperature difference between fluid elements exchanging heat such that entropy generation in the first and second heat storage units is minimized.
 29. The method of claim 27, wherein, during operation, the entropy generated in the first and second heat storage units is less than the entropy generated by the compressor.
 30. The method of claim 27, wherein the compressor and the turbine are operably coupled such that they rotate together.
 31. The method of claim 27, wherein the working fluid is a gas. 